Add "cmd" description, minor improvements (#10436)

doc/contributing.rst

@@ -71,9 +71,10 @@ to create a file for import into another test only, use the prefix ``m``.
 At the beginning of every test is the expected behavior of the test.
 Possible keys are:
 
-- output: The expected output, most likely via ``echo``
+- cmd: A compilation command template e.g. "nim $target --threads:on $options $file"
+- output: The expected output (stdout + stderr), most likely via ``echo``
 - exitcode: Exit code of the test (via ``exit(number)``)
-- errormsg: The expected error message
+- errormsg: The expected compiler error message
 - file: The file the errormsg was produced at
 - line: The line the errormsg was produced at